Lubricating oil composition of mineral oil and polyester for wet brake or wet clutch

ABSTRACT

The present invention provides a lubricating oil composition comprising 97 to 60% by weight of mineral oil and 3 to 40% by weight of polyester, said mineral oil having a kinematic viscosity at 100° C. of 2 to 50 centistokes, a pour point of -5° to -30° C., a viscosity index of not less than 80 and % C A  of not more than 3. This lubricating oil composition is suitably used for lubrication of parts including a wet brake and a wet clutch, such as automatic transmissions and tractors. 
     The lubricating oil composition of the present invention has a suitable viscosity at high temperatures and further is low in low temperature viscosity. 
     Furthermore the lubricating oil composition of the present invention is excellent in friction characteristics, oxidation stability and also in seal rubber compatibility.

FIELD OF TECHNOLOGY

The present invention relates to a lubricating oil composition and more particularly to a lubricating oil composition which is suitably used for lubrication of parts including a wet brake or a wet clutch of automatic transmissions and tractors.

BACKGROUND TECHNOLOGY

Lubricating oil for wet brake or wet clutch which is used in lubrication of parts including a wet brake or a wet clutch is required to be low in low temperature viscosity in view of starting performance. In general, the low temperature viscosity of lubricating oil can be easily decreased by decreasing the viscosity of the total base oil. In this case, however, the viscosity of the lubricating oil is too low at high temperatures, thereby producing a problem that the lubrication performance is decreased and the lubricating oil is unsuitable for practical use.

Therefore a method of compounding viscosity index improvers such as polymers to the low viscosity base oil has been widely used. This method, however, fails to solve the above problem because such polymers undergo viscosity reduction under shearing.

The first object of the present invention is to provide a base oil which holds a constant viscosity at high temperatures as one of the characteristics thereof and which is low in low temperature viscosity. It is, of course, required for the base oil to be excellent in oxidation stability and also in rubber swelling properties (seal rubber compatibility).

The second object of the present invention is to provide a lubricating oil composition in which friction characteristics for wet brakes or wet clutches are increased by the base oil itself.

SUMMARY OF THE INVENTION

The present invention provides a lubricating oil composition comprising 97 to 60% by weight of mineral oil and 3 to 40% by weight of polyester, wherein the mineral oil has a kinematic viscosity at 100° C. of 2 to 50 centistokes (cSt), a pour point (as determined by JIS K-2269) of -5° to -30° C, a viscosity index (as determined by JIS K-2283) of not less than 80 and % C_(A) (content of aromatic hydrocarbon) of not more than 3.

The lubricating oil composition of the present invention has a suitable viscosity at high temperatures and further is low in low temperature viscosity.

Further, the lubricating oil composition of the present invention is excellent in friction characteristics.

In addition, the lubricating oil composition of the present invention is excellent in oxidation stability and also in seal rubber compatibility.

DETAILED DESCRIPTION

Mineral oil as the major component of the lubricating oil composition of the present invention has a kinematic viscosity at 100° C. of 2 to 50 cSt, preferably 5 to 30 cSt, a pour point of -5° to -30° C, preferably -7.5° to -30° C., a viscosity index of not less than 80, preferably not less than 85, and % C_(A) of not more than 3, preferably not more than 1, if the above physical values are not within the above defined ranges, the desired lubricating oil composition cannot be obtained.

From a viewpoint of oxidation stability, mineral oil having a sulfur content of not more than 0.01% by weight, especially not more than 0.001% by weight is preferred.

Mineral oil having the properties as described above can be obtained by refining to a high extent a distillate (having a boiling point under atmospheric pressure of 250° to 450° C.) which has been obtained by distilling a paraffin base crude oil or an intermediate base crude oil.

This distillate means an oil obtained either by atmospheric distillation of crude oil or by vacuum distillation of residual oil resulting from atmospheric distillation of crude oil. A method of high refining is not critical, and any of the methods (1) to (5) as described below can be employed.

(1) The distillate is subjected to hydrogenation treatment, or alternatively, after hydrogenation treatment, the distillate is subjected to alkali distillation or sulfuric acid washing (treating).

(2) The distillate is subjected to solvent refining treatment, or alternatively, after solvent refining treatment, the distillate is subjected to alkali distillation or sulfuric acid washing (treating).

(3) The distillate is subjected to hydrogenation treatment followed by second hydrogenation treatment.

(4) The distillate is subjected to hydrogenation treatment, then to second hydrogenation treatment, and further to third hydrogenation treatment.

(5) The distillate is subjected to hydrogenation treatment followed by second hydrogenation treatment, and further to alkali distillation or sulfuric acid washing (treating).

One of the methods will hereinafter be explained.

A crude starting material for lubricating oil is produced from paraffin base crude oil or intermediate base crude oil by the usual method and then is subjected to severe hydrogenation treatment. In this treatment, undesirable components, such as aromatics, for the lubricating oil fraction are removed or converted into useful components. Almost all of sulfur and nitrogen components are removed at the same time.

Such fractional distillation as to obtain the necessary viscosity is carried out by vacuum distillation. Then the known solvent dewaxing treatment is carried out so as to dewax to the pour point that the usual paraffin base oil has, that is, about -15° to -10° C.

After the dewaxing treatment, if necessary, hydrogenation is carried out to hydrogenate the major portion of aromatic components into saturated components, thereby increasing thermal and chemical stability of the base oil. Then, to obtain the desired pour point, dewaxing treatment is applied. For this treatment, a solvent dewaxing method and a deep dewaxing method can be employed.

Conditions for hydrogenation treatment vary with the properties, etc. of the feed oil. Usually, the reaction temperature is 200° to 480° C. and preferably 250° to 450° C., the hydrogen pressure is 5 to 300 kg/cm² and preferably 30 to 250 kg/cm², and the amount of hydrogen introduced (per kiloliter of the fed distillate) is 30 to 3,000 Nm³ and preferably 100 to 2,000 Nm³. In this hydrogenation treatment, there are used catalysts which are prepared by depositing catalyst components such as Groups VI, VIII group metals, preferably cobalt, nickel, molybdenum and tungsten on carriers such as alumina, silica, silica-alumina, zeolite, active carbon and bauxite using the known method. It is preferred that the catalyst be previously subjected to preliminary sulfurization.

As described above, after hydrogenation treatment, the distillate is subjected to various treatments. When second hydrogenation treatment or further third hydrogenation treatment is applied, the treatment may be carried out under conditions falling within the ranges as described above. Conditions at the first, second and third stage hydrogenation treatments may be the same or different. Usually the second hydrogenation treatment is carried out under more severe conditions than the first stage hydrogenation treatment, and the third stage hydrogenation treatment, under more severe conditions than the second stage hydrogenation treatment.

Alkali distillation is carried out as a step where small amounts of acidic substances are removed to improve the stability of distillate. In this alkali distillation, alkalis such as NaOH and KOH are added and vacuum distillation is conducted.

Sulfuric acid washing (treating) is generally carried out as a finishing step of oil products, in which aromatic hydrocarbons, especially polycyclic aromatic hydrocarbons, olefins, sulfur compounds, etc. are removed to improve the characteristics of distillate. For example, 0.5 to 5% by weight of concentrated sulfuric acid is added to the distillate, the treatment is carried out at a temperature ranging between room temperature and 60° C., and thereafter neutralization using NaOH, etc. is applied.

The aforementioned methods (1) to (5) to be employed in treatment of distillate comprise combinations of the operations as described above. Of these methods, the methods (1), (3) and (4) are particularly suitable.

The mineral oil having the properties as described above can be obtained by subjecting the treatment described above to the base oil. Further, that mineral oil can be subjected to the clay treatment.

Polyesters which are used as the other component in the present invention include hindered esters or dicarboxylic acid esters.

Hindered esters having a pour point of not more than -30° C., preferably not more than -40° C. as hindered esters are used. Those having a pour point exceeding -30° C. are not preferred because they increase the low temperature viscosity. From viewpoints of kinematic viscosity, viscosity index and pour point, the following hindered esters are preferred.

Polyols in which the β-carbon of alcohol is quaternary, such as neopentyl glycol, trimethylolpropane, trimethylolethane and-pentaerythritol. are used as the polyol component constituting the hindered esters. As fatty acids which form hindered esters in combination with the above polyols, straight chain or branched fatty acids having 3 to 18 carbon atoms, and preferably 4 to 14 carbon atoms, especially branched fatty acids are preferred. Representative examples are straight chain fatty acids such as hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid and decanoic acid, and branched fatty acids such as 2-ethylhexanoic acid, isooctanoic acid, isononanoic acid and isodecanoic acid.

In addition, mixed fatty acids composed mainly of fatty acids having 4 to 14 carbon atoms are preferred used. These branched fatty acids and mixed fatty acids increase low temperature fluidity.

As dicarboxylic acis esters, those having a pour point of not more than -30° C., (preferably not more than -40° C.) are used. Dicarboxylic acid esters having a pour point of more than -30° C. are not preferred because they increase the lower temperature viscosity. From viewpoints of kinematic viscosity, viscosity index and pour point, the following dicarboxylic acid esters are preferred.

Branched alcohols having 3 to 18 carbon atoms, especially 4 to 13 carbon atoms are preferred as the alcohol component to form dicarboxylic acid esters. Representative examples are isobutyl alcohol, isoamyl alcohol, isohexyl alcohol, isooctyl alcohol, isononyl alcohol, isodecyl alcohol and isotridecyl alcohol. As dibasic acids to form dicarboxylic acid esters in combination with the above alcohols, adipic acid; azelaic acid, sebacid acid, dodecane dicarboxyIic acid, and the like can be used.

The lubricating oil composition of the present invention comprises the aforementioned mineral oil and polyester. The lubricating oil composition comprises 97 to 60% by weight of mineral oil and 3 to 40% by weight of polyester, and preferably 90 to 70% by weight of mineral oil and 10 to 30% by weight of polyester. If the proportion of the polyester is less than 3% by weight, the effects resulting from addition of the polyester cannot be obtained. On the other hand, if the proportion of the polyester is in excess of 40% by weight, rubber swelling properties (seal rubber compatibility) and friction characteristics are undesirably decreased.

The lubricating oil composition of the present invention comprises the aforementioned components.

To the lubricating oil composition of the present invention, if desired, additives such as an antioxidant, a detergent-dispersant, a viscosity index improver, a defoaming agent, an extreme pressure agent and a pour point decreasing agent can be added. When the lubricating oil composition of the present invention is used as a lubricating oil for use in lubricating parts including a wet brake or wet clutch, a friction modifier such as reaction products of fatty acids and amines can be added thereto. As the anti-oxidant, those commonly used such as phenol base compounds, amine base compounds and zinc dithiophosphate can be used. Representative examples are 2,6-di-tert-butyl-4-methyl-phenol; 2,6-di-tert-butyl-4-ethylphenol; 4,4'-methylenebis(2,6-di-tert-butyl-phenol); phenyl-α-naphthylamine; dialkyldiphenyl-amine; zinc di-2-ethylhexyldithiophosphate, zinc diamyldithiocarbamate, and pinene pentasulfide.

Detergent-dispersants which can be used include as ashless base dispersant and a metal-based detergent. For example, alkenylsuccinic acid imide, sulphonates and phenates are preferred. Representative examples of such preferred compounds are polybutenylsuccinic acid imide, calcium sulphonate, barium sulphonate, calcium phenate, barium phenate and calcium salicylate.

Viscosity index improvers are not critical, and polymethacrylate, polybutene and so forth can be used as viscosity index improvers.

EXAMPLES 1 TO 6, AND COMPARATIVE EXAMPLES 1 TO 11

Mineral oils having the properties shown in Table 1 and polyesters having the properties shown in Table 2 were mixed in the fixed ratios shown in Table 3 to prepare lubricating oil compositions. These lubricating oil compositions were evaluated and the results are shown in Table 3. The testing methods are as follows.

Testing methods

(1) Kinematic Viscosity

Measured according to JIS K-2283.

(2) Brookfield (BF) Viscosity

Measured according to ASTM D2983-80.

(3) ISOT (Test for Oxidation Stability of Lubricating Oil for Internal Combustion Engine)

Measured according to JIS K-2514, 3-1 (165.5° C.×48 hours).

(4) SAE No. 2 Friction Test

Friction characteristics were evaluated by the use of a SAE No. 2 friction tester (produced by Greening Co., U.S.A.) under the following conditions:

[Test Condition]

Disc: Three paper discs for an automatic transmission made in Japan.

Plate: Four plates made of steel for an automatic transmission made in Japan.

Number of revolutions of motor: 3,000 rpm.

Oil Temperature: 100° C.

μ1200 means a dynamic friction coefficient at a number of rotations of 1,200 rpm and μ0 means a static friction coefficient at the time that the motor is stopped. (5) Aniline Point

Measured according to JIS K-2256.

(6) Seal Rubber Dipping Test

Measured according to JIS K-6301 under the following conditions.

Rubber: nitrile rubber (A727 produced by Japan Oil Seal Co., Ltd.)

Oil Temperature: 150° C.

Test Duration: 170 hours

COMPARATIVE EXAMPLE 12

Commercially available paraffin-based solvent refining oils were evaluated in the same manner as in Example 1. The results are shown in Table 3.

                                      TABLE 1                                      __________________________________________________________________________                    Properties                                                                                          Sulfur                                                    Kinematic     Pour   Content                                                   Viscosity                                                                               Viscosity                                                                           Point                                                                              %  (% by                                                     (@ 100° C., cSt)                                                                 Index                                                                               (°C.)                                                                       C.sub.A                                                                           weight)                                                                             Remarks                               __________________________________________________________________________     Present                                                                               Mineral Oil I                                                                          2.26      86  -10 0  0.0001                                                                              *1                                    Invention                                                                             Mineral Oil II                                                                         5.35     105  -17.5                                                                              0  0.0008                                                                              *1                                    Comparison                                                                            Mineral Oil III                                                                        4.00      95  -17.5                                                                              7.0                                                                               0.05 *2                                           Mineral Oil IV                                                                         5.15     103  -15.0                                                                              7.5                                                                               0.5  *3                                           Mineral Oil V                                                                          4.08     -2   -37.5                                                                              14.0                                                                              0.8  *4                                           Mineral Oil VI                                                                         9.03      43  -25.0                                                                              7.8                                                                               0.5  *5                                    __________________________________________________________________________      *1 Mineral oil obtained in the following manner was used.                      *2 Paraffin base solvent refining oil                                          *3 Paraffin base solvent refining oil                                          *4 Naphthene based oil                                                         *5 Naphthene based oil                                                   

Kuwait crude oil was subjected to atmospheric distillation followed by vacuum distillation. A fraction resulting from deasphalting of the fraction and residual oil as obtained above was used as the feed stock and was subjected to hydrogenation treatment under such severe conditions that the viscosity index of the dewaxed oil product (after the first dewaxing treatment) reached 100.

The product obtained by the above method was fractionated to produce two distillates having viscosities at, 100° C. of 2.3 cSt and 5.6 cSt.

These two distillates were further subjected to solvent dewaxing treatment. Conditions for this treatment were such that the pour point of dewaxed oil was -15° C.

Then the above dewaxed oil was further subjected to hydrogenation treatment so that the aromatic content (as measured by the gel chromatograph-method) was not more than 1.5% by weight.

                  TABLE 2                                                          ______________________________________                                                 Properties                                                                     Kinematic           Pour                                                       Viscosity  Viscosity                                                                               Point                                                      (@ 100° C., cSt)                                                                   Index    (°C.)                                                                           Remarks                                    ______________________________________                                         Polyester I                                                                              4.3          142      -50   *1                                       Polyester II                                                                             3.48         162      -70   *2                                       ______________________________________                                          *1 Unistar H334R (produced by Nipopn Yushi Co., Ltd.): Ester of                trimethylolpropane and mixed fatty acids having 6 to 12 carbon atoms.          *2 DINA (produced by Sanken Kako Co., Ltd.): Adipic acid diisononyl ester

                                      TABLE 3                                      __________________________________________________________________________                Example                                                                        1   2   3   4   5   6                                               __________________________________________________________________________     Composition (wt %)                                                             Mineral oil I                                                                             20  17  11  3   26  13                                              Mineral oil II                                                                            68  59  77  73  69  50                                              Mineral oil III                                                                           --  --  --  --  --  --                                              Mineral oil IV                                                                            --  --  --  --  --  --                                              Mineral oil V                                                                             --  --  --  --  --  --                                              Mineral oil VI                                                                            --  --  --  --  --                                                  Polyester I                                                                               12  24  --  --  --  37                                              Polyester II                                                                              --  --  12  24  7   --                                              Additive *1                                                                               10.0                                                                               10.0                                                                               10.0                                                                               10.0                                                                               10.0                                                                               10.0                                            (parts by weight)                                                              Additive *2                                                                               5   5   5   5   5   5                                               (parts by weight)                                                              Results                                                                        Kinematic Viscosity                                                                       6.92                                                                               7.06                                                                               7.00                                                                               7.05                                                                               7.05                                                                               7.08                                            (@ 100° C., cSt)                                                        BF Viscosity                                                                              17500                                                                              16600                                                                              14800                                                                              11200                                                                              19600                                                                              14000                                           (@ -40° C., cp)                                                         ISOT                                                                           Kinematic Viscosity                                                                       1.14                                                                               1.12                                                                               1.11                                                                               1.07                                                                               1.16                                                                               1.11                                            Ratio (@ 100° C.)                                                       Increase in Total                                                                         0.40                                                                               0.80                                                                               0.29                                                                               0.45                                                                               0.26                                                                               0.75                                            Acid Number                                                                    SAE No. 2                                                                      μ1200   0.134                                                                              0.132                                                                              0.135                                                                              0.131                                                                              0.130                                                                              0.130                                           μ0/μ1200                                                                            1.06                                                                               1.06                                                                               0.06                                                                               1.07                                                                               1.05                                                                               1.08                                            Aniline Point (°C.)                                                                96.1                                                                               87.5                                                                               95.8                                                                               86.6                                                                               100.8                                                                              82.0                                            Seal Rubber                                                                    Dipping Test                                                                   Weight Change Ratio                                                                       2.8 3.9 2.8 4.0 2.0 6.6                                             (%)                                                                            Volume Change Ratio                                                                       7.1 7.3 5.7 7.2 3.7 11.2                                            (%)                                                                            __________________________________________________________________________                Comparative Example                                                            1   2   3   4   5   6                                               __________________________________________________________________________     Composition (wt %)                                                             Mineral oil I                                                                             28  --  --  --  --  --                                              Mineral oil II                                                                            72  --  --  --  --  --                                              Mineral oil III                                                                           --  68  60  40  --  --                                              Mineral oil IV                                                                            --  32  28  48  --  --                                              Mineral oil V                                                                             --  --  --  --  100 87                                              Mineral oil VI                                                                            --  --  --  --   -- 1                                               Polyester I                                                                               --  --  12  --  --  12                                              Polyester II                                                                              --  --  --  12  --  --                                              Additive *1                                                                               10.0                                                                               10.0                                                                               10.0                                                                               10.0                                                                               10.0                                                                               10.0                                            (parts by weight)                                                              Additive *2                                                                               5   5   5   5   5   5                                               (parts by weight)                                                              Results                                                                        Kinematic Viscosity                                                                       7.06                                                                               6.97                                                                               6.94                                                                               7.00                                                                               6.92                                                                               7.02                                            (@ 100° C., cSt)                                                        BF Viscosity                                                                              26300                                                                              36900                                                                              23100                                                                              22500                                                                              78700                                                                              46300                                           (@ -40° C., cp)                                                         ISOT                                                                           Kinematic Viscosity                                                                       1.20                                                                               1.52                                                                               1.31                                                                               1.27                                                                               1.93                                                                               1.91                                            Ratio (@ 100° C.)                                                       Increase in Total                                                                         0.22                                                                               7.37                                                                               5.39                                                                               4.31                                                                               6.70                                                                               6.45                                            Acid Number                                                                    SAE No. 2                                                                      μ1200   0.123                                                                              0.122                                                                              0.133                                                                              0.134                                                                              0.120                                                                              0.133                                           μ0/μ1200                                                                            1.04                                                                               1.05                                                                               1.08                                                                               1.09                                                                               1.06                                                                               1.07                                            Aniline Point (°C.)                                                                105.0                                                                              95.0                                                                               86.9                                                                               89.0                                                                               76.3                                                                               70.6                                            Seal Rubber                                                                    Dipping Test                                                                   Weight Change Ratio                                                                       1.3 2.7 4.0 3.8 9.7 11.3                                            (%)                                                                            Volume Change Ratio                                                                       3.2 5.6 7.0 6.8 16.5                                                                               20.5                                            (%)                                                                            __________________________________________________________________________                Comparative Example                                                            7   8   9   10  11  12                                              __________________________________________________________________________     Composition (wt %)                                                             Mineral oil I                                                                             --  --  --  21  11  Commercially                                                                   Available                                                                      Oil                                             Mineral oil II                                                                            --  --  --  78  39  Commercially                                                                   Available                                                                      Oil                                             Mineral oil III                                                                           --  --  --  --  --  Commercially                                                                   Available                                                                      Oil                                             Mineral oil IV                                                                            --  --  --  --  --  Commercially                                                                   Available                                                                      Oil                                             Mineral oil V                                                                             84  --  --  --  --  Commercially                                                                   Available                                                                      Oil                                             Mineral oil VI                                                                            4   --  --  --  --  Commercially                                                                   Available                                                                      Oil                                             Polyester I                                                                               --  100 --  --  50  Commercially                                                                   Available                                                                      Oil                                             Polyester II                                                                              12  --  100 1   --  Commercially                                                                   Available                                                                      Oil                                             Additive *1                                                                               10.0                                                                               10.0                                                                               10.0                                                                               10.0                                                                               10.0                                                                               Commercially                                    (parts by weight)              Available                                                                      Oil                                             Additive *2                                                                               5   5   5   5   5   Commercially                                    (parts by weight)              Available                                                                      Oil                                             Results                                                                        Kinematic Viscosity                                                                       6.96                                                                               7.29                                                                               7.24                                                                               7.01                                                                               7.09                                                                               6.91                                            (@ 100° C., cSt)                                                        BF Viscosity                                                                              40100                                                                              6460                                                                               1930                                                                               26000                                                                              13000                                                                              42000                                           (@ -40° C., cp)                                                         ISOT                                                                           Kinematic Viscosity                                                                       1.81                                                                               1.09                                                                               1.05                                                                               1.19                                                                               1.10                                                                               1.32                                            Ratio (@ 100° C.)                                                       Increase in Total                                                                         6.21                                                                               0.49                                                                               0.71                                                                               0.25                                                                               0.60                                                                               1.20                                            Acid Number                                                                    SEA No. 2                                                                      μ1200   0.133                                                                              0.125                                                                              0.177                                                                              0.123                                                                              0.129                                                                              0.124                                           μ0/μ1200                                                                            1.09                                                                               1.10                                                                               1.12                                                                               1.05                                                                               1.09                                                                               1.31                                            Aniline Point                                                                             73.8                                                                               Not Not 104.1                                                                              71.0                                                                               95                                              (°C.)   more                                                                               more                                                                       than                                                                               than                                                                       room                                                                               room                                                                       tem-                                                                               tem-                                                                       per-                                                                               per-                                                                       ature                                                                              ature                                                       Seal Rubber                                                                    Dipping Test                                                                   Weight Change Ratio                                                                       10.8                                                                               16.3                                                                               24.1                                                                               1.5 11.5                                                                               2.8                                             (%)                                                                            Volume Change Ratio                                                                       18.3                                                                               27.0                                                                               40.8                                                                               2.9 21.1                                                                               5.7                                             (%)                                                                            __________________________________________________________________________      *1 Package type additive containing a detergentdispersant, an antioxidant      a friction modifier, a defoaming agent and the like.                           *2 Polymethacrylate type viscosity index improver                        

The following can be seen from the results shown in Table 3. In Comparative Examples 1, 2 and 5, the low temperature viscosities (@-40° C.) were 23,800 cp, 36,900 cp and 78,700 cp, respectively; that is, the requirement in practical use that the low temperature viscosity is not more than 20,000 cp is not satisfied. In Comparative Examples 2 and 5, and increase in total acid number of ISOT is large, showing that the deterioration is seriously large.

In Comparative Examples 3 and 4, and Comparative Examples 6 and 7, the total acid number of ISOT is large and further the low temperature viscosity is low. However, the requirement in practical use that the low temperature viscosity is not more than 20,000 cp is not satisfied. In Comparative Examples 8 and 9, the aniline point is low, and the weight and volume change ratios of rubber are large, demonstrating that the swelling and softening is large.

In Comparative Examples 10 and 11, the properties are not within the range defined in the present invention. If the proportion of polyester is too small as in Comparative Example 10, the requirement in practical use that the low temperature viscosity (@-40° C.) is not more than 20,000 cp is not satisfied. On the other hand, if the proportion of polyester is too large as in Comparative Example 11, the aniline point is low and further the weight and volume change ratio to rubber is large, demonstrating that the swelling and softening is large.

If commercially available oil is used as in Comparative Example 12, the low temperature viscosity (@-40° C.) is 42,000 cp, which fails to satisfy the requirement in practical use. Furthermore, friction characteristics are not sufficiently satisfactory.

On the contrary, in Examples 1 to 6, the low temperature viscosity is not more than 20,000 cp, and oxidation stability (ISOT) and rubber swelling properties (seal rubber compatibility) are good. Furthermore, friction characteristics are excellent.

Possibility of Industrial Utilization

The lubricating oil composition of the present invention is suitable as a lubricant additive for parts including a wet brake and a wet clutch. For example, it can be used as a lubricant additive for automatic transmission fluid and a tractor oil. In addition, the lubricating oil composition of the present invention can be used as a power steering oil, an hydraulic oil or an internal combustion engine oil because it is low in low temperature viscosity and is good in oxidation stability and rubber swelling properties (seal rubber compatibility). 

I claim:
 1. A lubricating oil composition comprising 97 to 60% by weight of mineral oil and 3 to 40% by weight of polyester, said mineral oil having been subjected to high refining comprising a first hydrogenation treatment optionally followed by an alkali distillation or sulfuric acid washing; or the first and a second hydrogenation treatment optionally followed by an alkali distillation or sulfuric acid washing; or the first, the second and a third hydrogenation treatment; wherein said mineral oil has a kinematic viscosity at 100° C. of 2 to 50 centistokes, a pour point of -5° to -30° C, a viscosity index of not less than 80 and % C_(A) of not more than 3, and wherein the polyester is a hindered ester or a dicarboxylic acid ester.
 2. The composition as claimed in claim 1 wherein the mineral oil has a sulfur content of not more than 0.01% by weight.
 3. The composition as claimed in claim 1 wherein the mineral oil has a sulfur content of not more than 0.001% by weight.
 4. The composition as claimed in claim 1 wherein the mineral oil has a kinematic viscosity at 100° C. of 5 to 30 centistokes.
 5. The composition as claimed in claim 1 wherein the mineral oil has a pour point of -7.5° to -30° C.
 6. The composition as claimed in claim 1 wherein the mineral oil has a viscosity index of not less than
 85. 7. The composition as claimed in claim 1 wherein the mineral oil has % C_(A) of not more than
 1. 8. The composition as claimed in claim 1 wherein the mineral oil has a kinematic viscosity at 100° C. of 5 to 30 centistokes, a pour point of -7.5° to -30° C., a viscosity index of not less than 85 and % C_(A) of not more than
 1. 9. The composition as claimed in claim 1 wherein the polyester has a pour point of not more than -30° C.
 10. The composition as claimed in claim 1 wherein the polyester is a dicarboxylic acid ester of C₃ -C₁₈ alcohol and a C.sub. -C₁₄ dibasic acid.
 11. The composition as claimed in claim 10, wherein said C₃ -C₁₈ alcohol is isobutyl alcohol, isoamyl alcohol, isohexyl alcohol, isooctyl alcohol, isononyl alcohol, isodecyl alcohol or isotridecyl alcohol and said dibasic acid is adipic acid, azelaic acid or sebacic acid or dodecane dicarbolylic acid.
 12. The composition of claim 10 wherein said alcohol is a C₄ -C₁₃ alcohol.
 13. The composition as claimed in claim 8 wherein the polyester is a dicarboxylic acid ester of C₃ -C₁₈ alcohol and a C₆ -C₁₄ dibasic acid.
 14. The composition as claimed in claim 13, wherein said C₃ -C₁₈ alcohol is isobutyl alcohol, isoamyl alcohol, isohexyl alcohol, isooctyl alcohol, isononyl alcohol, isodecyl alcohol or isotridecyl alcohol and said dibasic acid is adipic acid, azelaic acid, sebacic acid or dodecane dicarboxylic acid.
 15. The composition of claim 13 wherein said alcohol is a C₄ -C₁₃ alcohol.
 16. The composition of claim 1 wherein the polyester is a hindered ester having a pour point of not more than -40° C.
 17. The composition of claim 1 wherein the polyester is the hindered ester.
 18. The composition of claim 1 wherein the polyester is the hindered ester of a polyol component and a fatty acid component, the polyol component is one wherein the β-carbon of alcohol is quaternary and the fatty acid component is a fatty acid having 3 to 18 carbon atoms.
 19. The composition of claim 18 wherein the fatty acid is hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, 2-ethylhexanoic acid, isooctanoic acid, isononanoic acid or isodecanoic acid.
 20. The composition of claim 18 wherein the fatty acid is a mixed fatty acid of C₄ -C₁₄ carbon atoms.
 21. The composition of claim 20 wherein the polyol component is neopentyl glycol, trimethylolpropane, trimethylolethane or pentaerythritol.
 22. The composition of claim 19 wherein the polyol component is neopentyl glycol, trimethylolpropane, trimethylolethane or pentaerythritol.
 23. The composition of claim 18 wherein the polyol component is neopentyl glycol, trimethylolpropane, trimethylolethane or pentaerythritol. 